Armor-all has silicone. Silicone displaces the original plasticizers in many plastics. So, once you go down that road you are married. I do not like it on vinyl coated convertible top fabric as eventually if too much too often is applied the backing fabric releases from the vinyl layer. I did have good luck on a '79 new car on the dash, but I was periodic, like every 3-5 months, no cracks after 11 years, but I knew I was "married" to it. Guy who bought it could not believe the dash was intact. Be careful on new cars, there is a new resin of choice, very unhappy with armor-all.
The 303 brand protectant... the MSDS sheet tells Nothing of use. I shy away from that as I want to know the chemistry.
Delrin is a brand name of nylon, nylon is hydroscopic, it will absorb water up to 3% of its mass and swell. That was a big headache for Toyota a number of years ago on Camry sticky accelerator assemblies.
Mineral oil such as "baby oil" will probably do as good a job as many elixirs, there are a few "back to black" products in the car care realm, some use a dye, a reasonable chance some of that black dye will penetrate into the plastic.
The grey is from oxidation and UV degradation, choose your dose, it has been decades.
The constant tension the plastic front derailleur bodies are under is not a well designed use of the plastic, yes delrin is self lubricating to a point but the expected use was under compression, not tension.
I have lost a few front derailleur bodies with the assembled bike just sitting in the bike room, yes, I did not know the bike's entire history but I don't trust that application much now at all.